If you set it up with a boolean value list then:
checked = 1
The value is dependent upon the data in the value list to create the checkbox set.
The only way to tell what the value is in the field, is to look at the value list attached to the checkbox in the field and verify it's values. You could also put another copy of the field on the layout without the checkbox attached, and look at the values in the field.
If you are using FileMaker Pro Advanced, you can check the values in the field using the Data Viewer.
Otherwise - go to layout mode, select the field, and check the field attributes in the Inspector. That will tell you the name of the value list assigned to the field.
THEN - go to File -> Manage -> Value Lists and check the value list name for the checkbox. It could be a number value, like 1, as Matt suggested, or it could be a text value, like "billable" If you are checking with a script step, then you need to know what it is, exactly.
Also, be aware that changing the value list will not change the value in the field for any records that have already been checked.
What is the value of a single checkbox if checked?
The value of any checkbox is a return-separated list of the checked items, in the order they were checked.